In recent years, attention has focused on preferred degradable polymers, which can be converted to desired substrates or articles. Much of this attention is focused on polymers which include, as monomeric units therein, the result of lactic acid or lactide polymerization. Attention is directed, for example, to U.S. Pat. No. 5,142,023 to Gruber et al.; U.S. Pat. No. 5,338,822 to Gruber et al.; U.S. Pat. No. 5,475,080 to Gruber et al.; U.S. Pat. No. 5,359,026 to Gruber; and U.S. Pat. No. 5,594,095 to Gruber et al. the complete disclosures of which are incorporated herein by reference. It is noted that U.S. Pat. Nos. 5,142,023; 5,338,822; 5,475,080; 5,359,026; and 5,594,095 are owned by Cargill Incorporated, of Minneapolis, Minn. Cargill Incorporated is the assignee of the present application as well.
Other published documents which concern polymers of lactic acid or lactide include: International Publication No. WO 94/06856 to Sinclair et al., published Mar. 31, 1994; International Publication No. WO 92/04413 to Sinclair et al., published Mar. 19, 1992; and International Publication No. WO90/01521 to Sinclair et al., published Feb. 22, 1990.
Paper coated with either polymeric or wax coatings is desirable because it may increase the strength of the paper stock, impart water resistance, enhance gloss, and/or improve barrier properties. Polypropylene is a common polymer used in paper coating processes. See Film Extrusion Manual, TAPPI Press, 1992, ISBN:0-89852-250-1.
In light of depleting sources of cellulosic fiber over the last decade, repulping of paper and the reuse of the cellulosic fiber recovered in the repulping process has accelerated. A typical repulping process involves mechanical agitation of the paper. Often the repulsing environment involves water, heat or other harsh conditions such as an acidic or alkaline solution. A problem that occurs with repulping coated paper is the disposal or recycling of the coating which is liberated during the repulp process. Furthermore, papers coated with certain polymers, such as, polyethylene, are not easily repulped since polyethylene is typically not broken down by the conditions of the repulping process.
Coatings have been developed which are represented to be "repulpable." These are materials which purportedly have adequate properties as paper coatings, and when exposed to conditions of repulping, either dissolve or disperse. In a solution or dispersion, it is claimed that these materials will pass through screens and other filtering steps and pass out with the waste water before the repulping step. Although these coatings have been extensively used, many problems have been encountered with their use. Often the coatings are not clear or glossy. Some coatings may also be unduly sensitive to water.
Disposal is a major problem associated with both repulpable and non-repulpable coating. For coatings which are recovered during the repulp process, there is no value in the recovered material if it must be disposed of in a landfill. For the coatings which pass through the filters and screens in the repulp process, these materials end up in the waste water and may pose a problem for the waste water treatment plants.
Polylactide polymers have been used to coat paper products. See U.S. Pat. No. 5,475,080. Polylactide polymers are advantageous because, once they are separated from the paper, they can be composted. Alternatively, the entire coated paper product can be composted. In order to meet projected needs for biodegradable packaging materials, others have endeavored to optimize lactide polymer processing systems. Gruber et al. (U.S. Pat. No. 5,142,023) disclose a continuous process for the manufacture of lactide polymers with controlled optical purity from lactic acid having physical properties suitable for replacing present petrochemical-based polymers.
Generally, manufacturers of polymers utilizing processes such as those disclosed by Gruber et al. will convert raw material monomers into polymer beads, resins or other pelletized or powdered products. The polymer in this forms is then sold to end users who convert, i.e., extrude, blow-mold, cast films, blow films, thermoform, injection-mold or fiber-spin the polymer at elevated temperatures to form useful articles. The above processes are collectively referred to as melt-processing. Polymers produced by processes such as those disclosed by Gruber et al., which are to be sold commercially as beads, resins, powders or other non-finished solid forms are generally referred to collectively as polymer resins.